Appendix C: Class Notes Example: Surface Tension in Aveoli / Lung Collapse

Recall that the "Laplace pressure" describes the difference in pressure between the inside (Pinside) and outside (Poutside) of a droplet due to surface tension, s. Previously we derived a relation showing that the difference in pressure determines the radius r of the drop:

Consider two joined air bubbles in liquid. There is a uniform pressure outside the bubbles.

What happens (can you guess?)

The Laplace pressures are:

The pressure inside bubble 2 (with respect to some common exterior pressure) is greater than in bubble 1, so air will flow from bubble 2 to bubble 1 until the second bubble disappears entirely. In foams this is called "coarsening".

Aveoli, the small air sacs in lungs, act as connected bubbles of radius 0.1-0.5mm in a liquid.

If the aveolar surface were simply an air-water interface, the change in Laplace pressure would cause small aveoli to coarsen, leading to lung collapse. Nature solves this problem by adding a layer of surfactant (lipids and protein) to the aveolar surface. The surfactant layer greatly reduces s, which is proportional to the concentration of surfactants at the aveolar surface.

Lung surfactant is compromised is smokers and in premature babies. The age limit at which premature babies live is related to when they can produce lung surfactant. This is an active area of research, especially to produce a synthetic lung surfactant. Currently, many therapies use lung surfactant derived from cows and there are concerns about transferring diseases among species.

References to Current Literature:

Enhancement of dendrimer-mediated transfection using synthetic lung surfactant exosurf neonatal in vitro,

KukowskaLatallo JF et al. BIOCHEM BIOPHYS RES COMM , v. 264(#1) pp. 253-261 OCT 14, 1999

Synthesis and structural characterization of human-identical lung surfactant SP-C protein

MayerFligge P et al. JOURNAL OF PEPTIDE SCIENCE , v. 4(#5) pp. 355-363 AUG 1998